Recently, CPUs (Central Processing Units) with high frequency and relatively high consumption power have become widespread. Personal computers called portable mobile computers and notebook personal computers (hereinafter referred to as notebook PCs) have also become widespread. For the purpose of portability, notebook PCs are powered by not only an AC power supply but also a battery.
However, if such a CPU with high frequency and high consumption power is used in a notebook PC, it is necessary to reduce the consumption power to extend the battery driving time. In one known method for reducing the consumption power, for example, it is determined whether a power supply voltage is supplied from an AC power supply or is supplied only from a battery, and the CPU clock frequency is lowered to reduce the consumption power if it is determined that a power supply voltage is supplied only from the battery.
Lowering the CPU clock frequency includes reducing the clock frequency itself, and conducting so-called throttling or driving and not driving the CPU (that is, intermittently driving the CPU to reduce the average frequency) to thereby equivalently reduce the frequency.
In activating the above-described function of dynamically changing the CPU frequency (hereinafter referred to as frequency transition (control)), communication performed by a modem can be interrupted, which is troublesome. Such communication interruptions can be caused by, for example, accumulating several hundred milliseconds for frequency transition control as a delay time. Thus, frequency transition control is set inactive during communication performed by a modem.
In the description herein, it is assumed that the CPU frequency is set to a high frequency when a power supply voltage is supplied from an AC power supply, and the CPU frequency is set to a low frequency when a power supply voltage is supplied from a battery.
During communication performed by a modem, if a power supply voltage is supplied from an AC power supply, the CPU frequency is set to a high frequency and frequency transition control is set to be inactive. In this state, even when the supply of power supply voltage is switched from the AC power supply to a battery, the frequency transition control is still inactive, and therefore the CPU frequency is still maintained at a high frequency.
Since the CPU frequency is still high although a power supply voltage is supplied from the battery, a problem occurs in that the operating time of the personal computer is shortened.
Conversely, during communication performed by the modem, if a power supply voltage is supplied from the battery, the CPU frequency is set to a low frequency and frequency transition control is set to be inactive. In this state, even when the supply of power supply voltage is switched from the battery to the AC power supply, frequency transition control is still inactive, and therefore the CPU frequency is still maintained at a low frequency.
Since the CPU frequency is still low although a power supply voltage is supplied from the AC power supply, a problem occurs in that the processing speed of the personal computer becomes low.
That is, a problem occurs is that even when the source of power supply voltage is changed during communication performed by the modem, the CPU frequency is not set (transitioned) to a suitable frequency for the current state. Such a problem can be experienced not only during communication performed by the modem but also in performing frequency transition control.